Add an emitSound() JS function

[DARwins1]

Adds a simple JS api function emitSound(), which plays a given sound file at the specified in-game coordinates.
E.g.

emitSound("lrgcan.ogg", 25, 50); // Plays the Heavy Cannon firing sound at x = 25, y = 50

EDIT: Coordinate arguments are now in whole tiles

[aubergineagain]

Tile units pls. The conversion to world units can be done in the c++ side.

[aco4]

1. Why tile units instead of world coordinates? Scripts will lose fine-grained control.
2. If tile units is preferred, then the sound should be emitted from the center of the tile, not the corner.